Optical multiplexer and demultiplexer

ABSTRACT

An optical multiplexer and demultiplexer (mux-demux) ( 100 ) comprises a multimode waveguide ( 126 ) which communicates with first ( 122 ) and second ( 124 ) coupling waveguides. Multiplexed optical radiation comprising individual wavelength channels of appropriate wavelength introduced into the input waveguide is demultiplexed by means of modal dispersion and in-ter-modal interference with the multimode waveguide. The mux-demux consists of merely of waveguides and is therefore simple to fabricate and integrate with other components in integrated optical systems, and is capable of resolving channels having a small (˜1 nm) wavelength spacing. The mux-demux may be used without modification as a demultiplexer and remains of simple construction when scaled up to operate with many channels.

The present invention relates to optical multiplexers and demultiplexers(mux-demuxes).

Optical multiplexing and demultiplexing, that is, combination andseparation of individual optical channels of various wavelengths intoand from a single (multiplexed) signal comprising those channels, is animportant function in optical communications systems. Multiplexing anddemultiplexing are typically performed within optical communicationssystems by array waveguide gratings (AWGs). An AWG is a devicecomprising a series of waveguides of different length each of whichcommunicates at one end with an input waveguide. For a given spectralcomponent within radiation input to the AWG, a phase variation acrossthe ends of the waveguides remote from the input waveguide is produced,the variation being specific to that spectral component. This allowsdifferent spectral components in the input radiation to be passed todifferent output waveguides of the AWG, thus achieving a demultiplexingfunction.

AWGs are described, for example, In the book“Optical Networks - APractical Perspective” by R. Ramaswami and K. N. Sivarajan (MorganKaufmann Publishers 1998, ISBN1-55860-445-6). They are complicateddevices requiring substantial processing effort In their fabrication,and are therefore time-consuming and expensive to produce. Furthermoretheir complexity makes it difficult to integrate them with other devices(e.g. lasers, modulators etc) within integrated optical systems.

Mux-demuxes based on the principle of self-imaging by modal dispersionand inter-modal interference within a multimode waveguide are of simplerconstruction than AWGs and hence provide for simpler fabrication andintegration. Two such devices are described in U.S. Pat. No. 5,862,288.A disadvantage with such devices is that the wavelengths at which theyoperate are constrained. For example, U.S. Pat. No. 5,862,288 describestwo mux-demuxes each of which operates to resolve (or combine) twooptical channels having wavelengths λ₁,λ₂. One device requires λ₂=2λ₁ inorder to operate and the other requires λ₂=2Mλ₁ where M is an integer.Such constraints on operating wavelengths mean that mux-demuxes of thistype are not suitable for use In practical WDM communication systems, inwhich optical channels have a wavelength spacing on the order of 1 nm,even though they are desirable from the point of view of simplefabrication and integration. Furthermore such devices become morecomplex in construction when designed to operate with many opticalchannels.

It is an object of the present invention to provide a mux-demux based onthe principle of self-imaging by modal dispersion and inter-modalinterference within a multimode waveguide and which is capable ofresolving optical channels having a wavelength spacing of a sizetypically found in practical optical communication systems.

According to a first aspect of the present invention, this object isachieved by an optical multiplexer and demultiplexer comprising

-   -   (i) a multimode waveguide;    -   (ii) a first coupling waveguide which communicates with the        multimode waveguide at a first longitudinal position therealong;        and    -   (iii) two second coupling waveguides which communicate with the        multimode waveguide at respective second longitudinal positions        therealong;        wherein the second longitudinal positions and the relative        orientations of the waveguides' central longitudinal axes are        such that an input optical field distribution, being a lowest        order transverse mode of the coupling waveguides and comprising        radiation of first and second wavelengths, when introduced into        the multimode waveguide via the first coupling waveguide is        substantially reproduced at the second longitudinal positions as        first and second output optical field distributions of first and        second wavelengths respectively, which output distributions are        coupled into respective second coupling waveguides, by virtue of        modal dispersion and inter-modal interference within the        multimode waveguide, characterised in that the coupling        waveguides each communicate with a lateral side of the multimode        waveguide.

The second longitudinal positions may be located on a lateral side ofthe multimode waveguide opposite to that on which the first longitudinalposition is located, in which case each second longitudinal position maybe separated from the first longitudinal position by a distance 4 mw²/λwhere m is a positive integer, w is the coupling waveguides' width and λis a wavelength to be multiplexed or demultiplexed.

Alternatively the first and second longitudinal positions may be locatedon a common lateral side of the multimode waveguide, in which case eachsecond longitudinal position may be separated from the firstlongitudinal position by a distance 8 mw²/λ where m is a positiveinteger, w is the coupling waveguides' width and λ is a wavelength to bemultiplexed or demultiplexed.

Alternatively the second longitudinal positions may be located on bothlateral sides of the multimode waveguide.

According to a second aspect of the present invention, there is provideda laser oscillator characterised in that it comprises a multiplexer anddemultiplexer according to the first aspect of the invention.

Embodiments of the invention are described below, by way of exampleonly, with reference to the accompanying drawings in which:

FIG. 1 shows a plan view of an optical multiplexer and demultiplexer ofthe invention;

FIGS. 2 and 3 illustrate the spatial distribution of an optical field asa function of distance within portions of the FIG. 1 multiplexer anddemultiplexer;

FIG. 4 is a plan view of another optical multiplexer and demultiplexerof the invention;

FIGS. 5 to 6 illustrate the spatial distribution of an optical field asa function of distance within portions of the FIGS. 4 multiplexer anddemultiplexer, and

FIG. 7 shows a plan view of a further optical multiplexer anddemultiplexer of the invention.

Referring now to FIG. 1, there is shown a plan view of a semiconductormultiplexer and demultiplexer (hereinafter “mux-demux”) of theinvention, indicated generally by 100 which has a central longitudinalaxis 101, and is referred to a coordinate system 111, which operates todemultiplex input radiation comprising three spectral components havingwavelengths within the mux-demux 100 of λ₁=1003 nm, λ₂=1000 nm andλ₃=997 nm. The mux-demux 100 has an input waveguide 122 and outputwaveguides 124A, 124B, 124C which communicate with a multimode waveguide126 of the mux-demux 100, meeting the multimode waveguide 126 onopposite lateral sides 127A, 127B thereof. The input and outputwaveguides 122, 124 have central axes inclined to the axis 101 at anangle α=42.9°. The input waveguide 122 communicates with the multimodewaveguide 126 at a point 123 and the output waveguides 124A, 124B, 124Ccommunicate with the multimode waveguide 126 at points 125A, 125B, 125C.The multimode waveguide 126 has a central longitudinal axis 101.

The input 122 and output waveguides 124A, 124B, 124C are each of widthw₁=2 μm. The multimode waveguide 126 has a width w₂=20 μm. The outputwaveguides 124A, 124B, 124C have respective centres 125A, 125B, 125C atthe multimode waveguide 126 which are separated in the z-direction fromthe centre 123 of the input waveguide 122 at the multimode waveguide 126by distances of L₁=4w₂ ²/λ₁=1595.2 μm, L₂=4W₂ ²/λ₂=1600.0 μm and L₃=4w₂²/λ₃=1604.8 μm respectively, i.e. centres of adjacent output waveguidesare separated in the z-direction by a distance of 4.8 μm.

Referring to FIG. 1A, there is shown a vertical section through themux-demux 100 along an xy plane |-| indicated in FIG. 1. In thex-direction the mux-demux 100 is a single-mode slab waveguide having aGaAs core layer 108 1 μm thick and Al_(0.1)Ga_(0.9)As cladding layers109, 106 having thicknesses of 2 μm and 4 μm respectively. Thewaveguides 122, 124, 126 are formed by etching through the core layer108 and into the cladding layer 106 to a depth of 2 μm to produce ridgestructures such as 112.

The mux-demux 100 operates as follows. Multiplexed input radiationcomprising optical channels having wavelengths of λ₁=1003 nm, λ₂=1000 nmand λ₃=997 nm within the mux-demux 100 is introduced into the inputwaveguide 122 of the mux-demux 300 and is guided therein as asingle-mode optical field. The input radiation enters the multimodewaveguide 126 at an xy plane 133. The spectral component of the inputradiation having wavelength λ₂=1000 nm excites transverse modes of theform EH_(1, j) at that wavelength within the multimode waveguide 126where j is an integer which may be either odd or even, I.e. bothsymmetric and antisymmetric transverse modes of the multimode waveguide126 are excited. As a result of modal dispersion and inter-modalinterference within the multimode waveguide 126, the input opticaldistribution in the y-direction of the spectral component λ₂=1000 nmevolves in the z-direction as shown in FIGS. 2 and 3.

Referring to FIG. 2, the intensity distribution in the y-direction ofthe spectral component λ₂=1000nm within the multimode waveguide 126 isshown at 5 μm intervals in the z-direction, from z=0 to z=40 μm measuredfrom the xy plane 133. The intensity distribution in the y-direction atthe xy plane 133 (z=0) is indicated in FIG. 2 by 140. The wavevector oflight within the multimode waveguide is indicated in FIG. 2 by k, whichis directed along the input waveguide axis 122A and is inclined at 41.9°to the axis 101.

Referring to FIG. 3, the intensity distribution in the y-direction ofthe spectral component λ₂=1000 nm is shown at 5 μm intervals in thez-direction from z=1580 μm to z=1600 μm. At a distance z=1600 μm amirror image 141 of the distribution 140 about the central axis 101 ofthe multimode waveguide 326 is produced as a result of modal dispersionand inter-modal interference within the waveguide 326. Light at the xyplane 135B has a wavevector k directed along the waveguide 324B andhence the spectral component λ₂=1000 nm is efficiently coupled into theoutput waveguide 324B.

Similarly, spectral component λ₁=1003 nm is coupled efficiently intooutput waveguide 324A because a mirror image of the input fielddistribution for that spectral component is generated about the axis 101at a distance L₁ from the xy plane 133. Spectral component λ₃=997 nm isefficiently coupled into output waveguide 324C because a mirror image ofthe input field distribution for that spectral component is generatedabout the axis 101 at a distance L₃ from the xy plane 133. The mux-demux100 thus efficiently demultiplexes the spectral components λ₁, λ₂, λ₃which are combined in the input radiation which is introduced into theinput waveguide 122.

The angle α may take values other than 42.90°, however it must besufficiently small to allow total internal reflection of light withinthe multimode waveguide 128. In the present case, the angle α must beless than 73.3°. The angle α must also be sufficiently large to avoidphase perturbation effects of modes within the multimode waveguide 126.

Referring now to FIG. 4 there is shown another mux-demux of theinvention, indicated generally by 200 and referred to a coordinatesystem 211. The mux-demux 200 also operates to demultiplex inputradiation comprising three spectral components having wavelengths withinthe mux-demux 200 of λ₁=1003 nm, λ₂=1000 nm and λ₃=997 nm. The mux-demux200 has an input waveguide 222 and output waveguides 224A, 224B, 224Cwhich communicate with a multimode waveguide 226 having lateral sides227A, 2278, meeting the multimode waveguide 226 on a lateral side 227Athereof at an angle α=42.9°. The structure of the mux-demux 200 in thex-direction is like to that of the mux-demux 100 of FIG. 1. The input222 and output waveguides 224A, 224B, 224C are each of width w₁=2 μm.The multimode waveguide 226 has a width w₂=20 μm. The output waveguides224A, 224B, 224C have respective centres 225A, 225B, 225B at themultimode waveguide 226 which are separated in the z-direction from thecentre 223 of the input waveguide 222 at the multimode waveguide 226 bydistances of I₁=8w₂ ²/λ₁=3190.4 μm, I₂=8w₂ ²/λ₂=3200.0 μm and I₃=8w₂²/λ₃=3209.6 μm respectively, i.e. centres of adjacent output waveguidesare separated in the z-direction by a distance of 9.6 μm.

The mux-demux 200 operates in a like manner to the mux-demux 100.Multiplexed input radiation comprising optical channels havingwavelengths λ₁=1003 nm, λ₂=1000 nm and λ₃=997 nm within the mux-demux200 is introduced into the input waveguide 222 of the mux-demux 200 andis guided therein as a single-mode optical field. The input radiationenters the multimode waveguide 226 at an xy plane 233. The spectralcomponent λ₂=1000 nm of the input radiation excites transverse modes ofthe form EH_(1,j) at that wavelength within the multimode waveguide 226where j is an integer which may be either odd or even, i.e. bothsymmetric and antisymmteric transverse modes of the waveguide 226 areexcited. As a result of modal dispersion and inter-modal interferencewithin the multimode waveguide 226, the input optical distribution inthe y-direction of the spectral component λ₂=1000 nm evolves in thez-direction as shown in FIGS. 5 and 6.

Referring to FIG. 5, the intensity distribution of the spectralcomponent λ₂=1000 nm in the y-direction within the multimode waveguide226 is shown at 5 μm intervals in the z-direction, from z=0 to z=40 μmmeasured from the xy plane 233. The intensity distribution in they-direction at the xy plane 233 (z=0) is indicated in FIG. 5 by 240.Referring to FIG. 6, the intensity distribution in the y-direction ofthe spectral component λ₂=1000 nm is shown at 5 μm intervals in thez-direction from z=3180 μm to z=3200 μm. At a position z=3200 μm, anintensity distribution 241 is produced as a result of modal dispersionand inter-modal interference. The distribution 241 is substantially thesame as the distribution 240, although light at the xy plane 235B has awavevector k′ such that k′_(y)=−k_(y) and |k′|=|k |. The spectralcomponent λ₂ =1000 nm is therefore efficiently coupled into outputwaveguide 2248.

Similarly, spectral component λ₁=1003 nm is coupled efficiently intooutput waveguide 224A because the input field distribution for thatspectral component is reproduced at a distance I₁ from the xy plane 233.Spectral component λ₃=997 nm is coupled efficiently into outputwaveguide 224C because the input field distribution for that spectralcomponent is reproduced at a distance I₃ from the xy plane 233.

The mux-demux 200 thus efficiently demultiplexes the spectral componentsλ₁=1003 nm, λ₂=1000 nm and λ₃=997 nm which are combined in the inputradiation which is introduced into the input waveguide 222.

The input 122 and output 124 waveguides may be single-mode guides in theyz plane. Alternatively they may multimoded in the yz plane, in whichcase multiplexed signal light must be introduced into the inputwaveguide 122 such that only the lowest order transverse mode of thatwaveguide is excited.

If spectral components in the input radiation for mux-demuxes 100, 200are more closely spaced in wavelength than 3 nm, centres of the outputwaveguides 124, 224 must be more closely spaced in the z-direction.However for an output waveguide width w₁, centres 125, 225 of the outputwaveguides have a minimum separation in the z-direction of w₁/sin α=2.94μm as a result of finite width of the output waveguides: this places alower limit on the wavelength spacing of the optical channels which canbe demultiplexed by the mux-demuxes 100, 200.

The mux-demux 100 utilises the phenomenon of generation of a mirrorimage about a central longitudinal axis 101 of an input fielddistribution 140 of a spectral component λ at a distance L=4w₂ ²/λwithin the multimode waveguide 126, whereas the mux-demux 200 utilisesreplication of an input field distribution 240 of a spectral component λat a distance L=8w₂ ²/λ within the multimode waveguide 226. Therefore achange dλ in wavelength of a particular spectral component λ correspondsto a change in z-position of a corresponding output waveguide of (−4w₂²/λ²)dλ in the case of the mux-demux 100 and (−8w₂ ²/λ²)dλ in the caseof the mux-demux 200, i.e. the rate of change of z-position withwavelength of the centre of an output waveguide for the mux-demux 200 istwice that for the mux-demux 100. Hence a mux-demux such as 200 iscapable of greater wavelength resolution than a mux-demux such as 100.For example, if the output waveguides 124A, 124B, 124C of the mux-demux100 are arranged contiguously (i.e. without any intervening spaces) andL₂=4w₂ ²/λ₂=1600 μm (λ₂=1000 nm) then the mux-demux 100 would operate todemultiplex channels having a wavelength spacing${\Delta\quad\lambda} = {\frac{w_{1}\lambda_{2}^{2}}{4w_{2}^{2}\sin\quad\alpha} = {1.84\quad{nm}}}$i.e. to demultiplex channels having wavelengths λ₁=1001.84 nm, λ₂=1000nm, λ₃=998.16 nm.

If the output waveguides 224A, 224B, 224C of the mux-demux 200 were tobe arranged contiguously with L₂=8w₂ ²/λ=3200 μm (λ₂=1000 nm), themux-demux 200 would operate to demulitplex channels having a wavelengthspacing${{\Delta\quad\lambda} = {\frac{w_{1}\lambda_{2}^{2}}{8w_{2}^{2}\sin\quad\alpha} = {0.92\quad{nm}}}},$i.e. to demultiplex channels having wavelengths, λ₁=1000.92 nm, λ₂=1000nm, λ₃=998.08 nm.

Alternative mux-demuxes of the invention may be based on generation of amirror image about a central longitudinal axis of a multimode waveguideof an input field distribution of a spectral components λ in az-distance 4Nw₂ ²/λ (where N is an odd positive integer) within themultimode waveguide; input and output waveguides of such a device aredisposed on opposite lateral sides of a multimode waveguide, as in FIG.1. Further alternative mux-demuxes of the invention may be based onreplication of an input field distribution of a spectral component λ ina z-distance 4Nw₂ ²/λ (where N is an even integer) within a multimodewaveguide; input and output waveguides of such a device are disposed ona common lateral side of a multimode waveguide, as in FIG. 2.

Referring now to FIG. 7, there is shown a further mux-demux of theinvention, indicated generally by 300. Parts of the mux-demux 300equivalent to those of the demultiplexer 200 are like referenced withnumerals differing from those in FIG. 4 by a value of 100. The mux-demux300 is referred to a coordinate system 311 and has a construction liketo that of the mux-demux 200, except that one output waveguide, 324B, isdisposed on a lateral side of a multimode waveguide 326 opposite to thatwhich communicates with the input waveguide 322 and the other outputwaveguides 324A, 324C. The mux-demux 300 is arranged to demultiplexchannels having wavelengths λ₁=1003 nm, λ₂=1000 nm and λ₃=997 nm whichare introduced into the input waveguide 322 as a multiplexed opticalsignal. Centres 325A, 325B, 325C of output waveguides 324A, 324B, 324Cat the multimode waveguide 326 are displaced in the z-direction from thecentre 323 of the input waveguide 322 at the multimode waveguide 326 bydistances I₁=8w₂ ²/λ₃=3190.4 μm, L₂=4w₂ ²/λ₂=1600 μm and I₃=8w₂²/λ₃=3209.6 μm respectively. Individual demultiplexed optical channelsλ₁=1003 nm, λ₂=1000 nm and λ₃=997 nm exit the mux-demux 300 via outputwaveguides 324A, 324B and 324C respectively.

A mux-demux such as 300 provides an alternative to a device such as 200in circumstances where individual optical channels within the inputradiation are so closely spaced in wavelength that the output waveguidesof a mux-demux such as 200 are difficult or impossible to fabricatebecause of their dose spacing. A mux-demux such as 300 provides afurther increase in wavelength resolution over a device such as 200. Forexample, a variant of the device 300 in which L₂ =4w₂ ²/λ₂=1600 μm(λ₂=1000 nm), I₁=3198.5319 μm and is I₃=3201.4695 μm (i.e. centres 325A,325C of output waveguides 324A, 324C, are separated by a z-distance ofw₂/sin α=2.94 μm so that those output waveguides are contiguous in thez-direction) operates to demultiplex channels having a wavelengthspacing of 0.4590 nm, i.e. to demultiplex channels having wavelengthsλ₁=1000.4590 nm, λ₂=1000.0000 nm and λ₃=999.5410 nm.

Although the mux-demuxes described above each have three outputwaveguides, devices of the invention may have two or more waveguides andoperate to demultiplex an optical signal comprising two or moreindividual wavelength channels.

The devices 100, 200, 300 described above may be used in reverse tomultiplex optical channels, i.e. to combine optical signals of differentwavelength into a single optical signal. Suitable single-wavelengthsignals may be introduced into the waveguides 124, 224, 324 andmultiplexed signals then exit the devices via the waveguides 122, 222,322.

A mux-demux of the invention may be modified to produce an active (laseroscillator) device which generates output radiation comprisingmultiplexed wavelength channels. For example, the mux-demux 200 of FIG.4 may be modified by providing mirrors at the ends of the waveguides222, 224 and by providing optical gain at appropriate wavelengths withinthe waveguides 224A, 224B, 224C. Optical output is then obtained fromthe waveguide 222 in the form of multiplexed laser radiation consistingof wavelengths of λ₁=1003 nm, λ₂=1000 nm and λ₃=997 nm. If the laseroscillator's optical gain is provided by passing current through each ofthe waveguides 224, such a device may be also be used to modulate theindividual output channels as would be required in an opticalcommunication system. For example, the current applied to a particularwaveguide 224 may be switched between two values such that theround-trip gain within the device 200 for the wavelength channelcorresponding to that waveguide is switched above and below lasingthreshold.

1. An optical multiplexer and demultiplexer comprising (i) a multimodewaveguide (ii) a first coupling waveguide which communicates with themultimode waveguide at a first longitudinal position therealong; and(iii) two second coupling waveguides which communicate with themultimode waveguide at respective second longitudinal positionstherealong; wherein the second longitudinal positions and the relativeorientations of the waveguides' central longitudinal axes are such thatan input optical field distribution, being a lowest order transversemode of the coupling waveguides and comprising radiation of first andsecond wavelengths, when introduced into the multimode waveguide via thefirst coupling waveguide is substantially reproduced at the secondlongitudinal positions as first and second output optical fielddistributions of first and second wavelengths respectively, which outputdistributions are coupled into respective second coupling waveguides, byvirtue of modal dispersion and inter-modal interference within themultimode waveguide, characterised in that the coupling waveguides eachcommunicate with a lateral side of the multimode waveguide.
 2. Amultiplexer and demultiplexer according to claim 1 wherein the secondlongitudinal positions are located on a lateral side of the multimodewaveguide opposite to that on which the first longitudinal position islocated.
 3. A multiplexer and demultiplexer according to claim 2characterised in that each second longitudinal position is separatedfrom the first longitudinal position by a distance 4mw²/λ where m is apositive integer, w is the coupling waveguides' width and λ is awavelength to be multiplexed or demultiplexed.
 4. A multiplexer anddemultiplexer according to claim 1 characterised wherein the secondlongitudinal positions and the first longitudinal position are locatedon a common lateral side of the multimode waveguide.
 5. A multiplexerand demultiplexer according to claim 4 characterised in that each secondlongitudinal position is separated from the first longitudinal positionby a distance 8 mw²/λ where m is a positive integer, w is the couplingwaveguides' width and λ is a wavelength to be multiplexed ordemultiplexed.
 6. A multiplexer and demultiplexer according to claim 1wherein second longitudinal positions are located on both lateral sidesof the multimode waveguide.
 7. A laser oscillator characterised by amultiplexer and demultiplexer according to claim 1.